1. Field of the Invention
The invention relates to a turbomachine rotor stage of the type comprising a thin annular body including reinforcing rings, and particularly, but not exclusively, relates to a rotor stage of this type wherein the body and the blades carried thereby are formed as a single integral unit.
2. Summary of the Prior Art
In order to lighten the compressor rotors of aircraft jet engines and to increase their speed of rotation, rotors now take the form of a thin annular body reinforced by fibrous rings and supporting the blades on its outer periphery. This construction has made it possible to achieve very high speeds of rotation, but has two main disadvantages.
Firstly, rotors of this type have considerable radial flexibility due to the thinness of the annular body, which gives rise to vibrations, mainly in radial directions. These vibrations may occur in modes 1d, 2d, 3d and so on--i.e. with two diametrically opposite resonance antinodes in mode 1d, four resonance antinodes spaced 90.degree. apart from one another in mode 2d, six resonance antinodes spaced 60.degree. apart from one another in mode 3d, and so on. These vibrations can become very severe. They cause premature fatigue of the rotor material and thus reduce the working life of the rotor. The vibrations are also transmitted to the rest of the engine by way of the bearings, with the result that the bearings and the engine structure have to be reinforced, leading to increased engine cost and weight.
This phenomenon is aggravated by the presence of the blades on the periphery of the body, since the blades form an additional mass of material which does not contribute to rotor rigidity, causing the resonant frequencies of the rotor to be reduced and the amplitude of the vibrations to be correspondingly increased.
Secondly, the fibrous reinforcing rings are very expensive for two reasons. One is the high cost of the fibres used, which are generally of silicon carbide, SiC. The other is the actual construction of the fibrous rings, in which the fibres, whose diameter is of the order of 100 .mu.m, are spaced apart very evenly in a metal matrix, usually of TA6V titanium alloy. There are two categories of fibrous reinforcing rings which may be used.
In a first category the reinforcing rings consist of wound fibres which are spaced apart from the metal of the matrix before the assembly is compacted by hot pressing or forging along the geometric axis of the spirals. The reinforcing rings are then welded to the rotor, or disposed in the rotor before subjecting it to a high-temperature pressing cycle serving to shape the rotor and weld the rings to the rotor material.
Fibrous rings of this type usually have a rectangular cross-section in which adjacent sides have very similar lengths. The tensile strength in respect of circumferential stresses is very high since they are effectively in the direction of the fibres, but the tensile strength in respect of radial stresses is less since the stresses act perpendicularly to the fibres. Since the rings are of reduced width in the axial direction, their resistance to a radial force tending to separate the fibre layers is unsatisfactory. Such rings are therefore usually disposed around the material of the rotor (i.e. , at the rotor periphery), on the sides of the rotor, or inside the rotor near the centre of gravity of the rotor cross-section.
In the second category the reinforcing rings consist of a superposed stack of strips of matrix metal which are rolled into a cylindrical shape and parallel reinforcing fibres which cover each of the cylinders and are disposed in the circumferential direction of the cylinders. The stack of strips and fibres is disposed against the inner wall of a rotor blank and the assembly is treated by centrifugal radial forging to compact the strips and fibres and weld the resulting fibrous mass to the inner wall of the rotor. This arrangement is considered to be more rational to the extent that it helps to concentrate the reinforcing fibres against the inner wall of the rotor, which is the zone most stressed by the centrifugal force caused by rotation. It also makes it possible to have a reinforcing ring with a very long rectangular cross-section extending over the entire width of the rotor. The disadvantage of this arrangement, however, is that the fibres must be cut to the length of the circumference which they occupy before centrifugal forging if they are not to break during the radial centrifugal forging. Although these cuts are distributed uniformly so as not to overlap, they nevertheless produce non-uniformities which weaken the rotor.